Piston-type hydraulic actuators are used in numerous structural and material testing applications. For example, such actuators are used in the aerospace and automotive industries to test structural parts for fatigue. Additionally, these actuators may be used for aircraft braking systems, flight control systems, flight simulation systems or force control systems for industrial process equipment. A typical actuator of this type includes a movable fluid driven piston inside a cylinder having an actuator chamber on each side of the piston for moving the piston in opposite directions when pressurized fluid is supplied to one of the chambers.
In many applications, it is necessary that the actuator be controlled to apply a force to a structural part as a function of time, such as through a fatigue loading cycle. One system used for controlling the movement of an actuator is a flow control valve that includes a torque motor having a pivotable armature, a jet-pipe/receiver assembly (first stage) having a jet pipe connected to the armature and a pair of receivers, and a single four-way sliding spool valve (second stage) having a spool that controls two ports, one to each of the actuator chambers. Hydraulic fluid at system pressure is fed to a jet pipe nozzle that directs a fine jet stream of fluid at the two receivers. Each receiver is connected to a corresponding end of the second stage spool. At null (no signal to the torque motor), the jet stream impinges equally on each receiver and equal recovery pressure (approximately one-half system pressure) is generated in each. Thus, the forces at each end of the second stage spool are equal and it remains in the null position. When an electrical input signal is applied to the torque motor, it causes the armature and the jet-pipe to rotate about the armature pivot point, causing more fluid to impinge on one receiver than the other. The resulting differential pressure at the ends of the spool causes the spool to move, which opens the second stage ports, causing fluid to flow to one actuator chamber and out of the other actuator chamber. A feedback spring between the jet-pipe and the spool is used to counteract the force applied to the jet-pipe by the torque motor and stabilizes the spool in its new position.
One problem with flow control valves of this type is that they have an inherently high pressure gain or amplification, e.g., at about 3% of rated valve input current, approximately 90% of system pressure is obtained. Thus, if a structural test requires a maximum pressure level of 1% of system pressure, then the system must control down to 0.03% of rated current to obtain this pressure setting. This fine resolution of control is difficult to achieve with flow control valves given the present accuracy of the valves, transducers and control electronics. High gain also results in an abrupt transition or backlash of the piston when it reverses directions, possibly affecting the stability of the mechanical joints in the system being tested.
Flow control valves also have further disadvantages. Zero slack side pressure (i.e., zero pressure on the low pressure side of the actuator) cannot be achieved, and therefore, to obtain a given force on the structural part being tested, the actuator piston must overcome the slack side pressure, giving a load error to the control system. Because of slack side pressurization, complex abort manifolds incorporating expensive, specially designed, differential load limiters are required to provide a safety outlet in the event of overpressurization or other malfunction. Flow control valves also require load sensor feedback loops which result in channel crosstalk in large installations using many flow control valves.
Another system used for controlling the movement of an actuator is a pair of valves, one connected to each side of the actuator. Each valve has a torque motor, a nozzle/flapper receiver assembly (first stage) and a three-way sliding spool (second stage). Typically, one valve is energized at a time, pressurizing only one side of the actuator at a time. The system therefore does not have the disadvantages of the flow control valve, such as slack side pressurization. However, the dual valve system tends to be more expensive and to have greater maintenance requirements because it requires two torque motors and two nozzle/flapper receiver assemblies. The system also has the potential to pressurize both sides of the actuator at the same time if one valve fails. Furthermore, the nozzle/flapper receiver assembly is not as contamination tolerant as the jet-pipe type of assembly.
Another problem encountered with piston-type hydraulic actuators in general is the handling of unequal area actuators. Typically, a single piston rod is mounted at one end to the piston and at its other end to the structural load being tested. Because the piston rod is on one side only, the opposite faces of the piston have unequal areas being exposed to the pressurized fluid. The force transmitted by the piston is equal to the pressure in the actuator cylinder multiplied by the area of the piston face. Thus, the pressure applied to one side of the actuator will result in a different magnitude of force applied to the load than if the same pressure is applied to the other side of the cylinder. It is desirable that the force applied by the piston be the same and proportional to the control current applied to the valve, regardless of the direction of piston movement. Present methods of achieving this, however, involve the use of complex control electronics that are not easily corrected should it become necessary to make changes to the actuator in the field.
From the above, it should be appreciated that there is a need for a less complex hydraulic control system that smoothly controls the pressure applied to each side of the actuator and the movement of the actuator itself. Ideally, such a system would also correct the problem associated with unequal area actuators such that the piston force applied to the structure is generally proportional to the control current applied by the control system. The present invention provides the necessary solution.